Báo cáo hóa học: " Ultra violet sensors based on nanostructured ZnO spheres in network of nanowires: a novel approach" pdf

NANO EXPRESS
Ultra violet sensors based on nanostructured ZnO spheres
in network of nanowires: a novel approach
S. S. Hullavarad Æ N. V. Hullavarad Æ P. C. Karulkar Æ
A. Luykx Æ P. Valdivia
Published online: 3 March 2007
Ó to the authors 2007
Abstract The ZnO nanostructures consisting of micro
spheres in a network of nano wires were synthesized by
direct vapor phase method. X-ray Photoelectron
Spectroscopy measurements were carried out to
understand the chemical nature of the sample. ZnO
nanostructures exhibited band edge luminescence at
383 nm. The nanostructure based ZnO thin films were
used to fabricate UV sensors. The photoresponse
measurements were carried out and the responsivity
was measured to be 50 mA W
–1
. The rise and decay
time measurements were also measured.
Keywords UV Sensor Á Nano structures Á Micro-
spheres Á Nanowire network Á Rise/Decay time Á
Photoresponse Á Photoluminescence
Introduction
Zinc oxide (ZnO) is a promising wide bandgap semi-
conductor for applications in ultra violet (UV) light
emitting devices and sensors [1]. ZnO is a direct band-
gap (Eg = 3.37 eV) semiconductor with a large exciton
binding energy (60 meV) [2], exhibiting near UV
emission, transparent conductivity [3] and piezoelec-
tricity [4]. The high exciton binding energy in ZnO

crystal can ensure an efficient excitonic emission at
room temperature under low excitation intensity,
which has potential applications in high efficiency light
emitting diodes and UV lasers. ZnO has been widely
reported as a visible blind UV sensor [5] over a wide
range of applications in military and non-military are-
nas [6] that includes missile plume detection for hostile
missile tracking, flame sensors, UV source monitoring,
and calibration [7]. The research in the sensor area has
lead many researchers to explore the possibility of
widening the band gap of ZnO by alloying with Cd [8]
and Mg so as to cover UV-A, UV-B and UV-C region
of ultra violet region [9]. ZnO possess unique figures of
merit, such as low thin-film growth temperatures (100–
750 °C) [10], and radiation hardness [11], which are
crucial for practical optoelectronic devices. Despite the
challenges of reliable p-type doping in ZnO, there have
been reports on fabrication of photodetectors [12],
quantum wells[13], and superlattices [14] based on
ZnO. As was predicted [15], the observation of room-
temperature UV lasing from the ordered, nano-sized
ZnO crystals provides an important step for the
development of practical blue-UV laser. Thus, ZnO
nanoscale structures such as one-dimension nanowires
are attracting more attention because of their enor-
mous potential as fundamental building blocks for
nanoscale electronic [16] and photonic devices due to
enhanced sensitivity offered by quantum confinement
effects [17]. The sensors consisting of nanostructures
[18] with large surface area to volume ratio (spheres)

have better response characteristics and higher sensi-
tivity [19]. ZnO nanostructured thin films consisting of
spheres of diameter approximately 40–65 nm prepared
by sol–gel dip coating method for gas sensing applica-
tions have been reported [20]. Zhang et al. [21], have
S. S. Hullavarad (&) Á N. V. Hullavarad Á
P. C. Karulkar
Office of Electronic Miniaturization, University of Alaska
Fairbanks, Fairbanks, AK 99701, USA
e-mail:
A. Luykx Á P. Valdivia
Center for Superconductivity Research, University of
Maryland, College Park, MD 20742, USA
123
Nanoscale Res Lett (2007) 2:161–167
DOI 10.1007/s11671-007-9048-6
reported the fabrication of humidity sensors based on
ZnO nanorods and nanowires grown by vapor phase
transport process on pre patterned platinum electrode
substrates. The formation of different shapes of nano-
structures depends largely on temperature, pressure,
heating/cooling rates and the saturation of reactive
elements in the gaseous phase during the reaction.
There have been reports on fabrication of complex
ZnO structures with morphology like mushroom [22],
spheres [23], ellipsoids, flowers and propellers [24].
Luo et al. [25] have reported the fabrication of UV
photodiode by forming a heterojunction of n-ZnO
nanowires of diameter (70–120 nm) with p-type Si. The
authors have observed the UV responsivity of

70 mA W
–1
under UV illumination of 365 nm at
unusually higher (20 V) reverse bias. In this paper, we
report the fabrication of ZnO UV sensor consisting of
ZnO micro spheres in a matrix of nanowires.
Experimental
ZnO sheets consisting of spheres in nanowires were
synthesized in a horizontal tube furnace (Lindberg) by
a self catalyst Direct Vapor Phase (DVP) technique.
Figure 1 shows the schematic of the growth set up. The
source material Zn (99.9%) in granular form was
placed at the center of the furnace (800 °C, heating
rate 10 °C min
–1
). Double side polished Al
2
O
3
(0001)
samples were used as substrates for optical character-
ization. In the initial stage, the furnace was flushed by
Ar gas and was latter stabilized with a flow rate of 40–
50 sccm. When the furnace reaches 420 °C, the Zn
metal evaporates and O
2
gas was introduced with the
combined gas mixture of 60 sccm. The evaporated Zn
metal forms ZnO and was deposited on the Al
2

O
3
substrates and also on the walls of the tube furnace.
The process was carried out for 90 minutes and sam-
ples were pulled out after furnace was cooled down to
room temperature. ZnO nanostructures were charac-
terized by Environmental Scanning Electron Micros-
copy (Electro Scan), and Photoluminescence (PL)
measurements to monitor the morphology (Laser
Science, Inc, Model VSL-337 ND-S, 337 nm, 6 mW
and Ocean Optics SD5000 spectrometer) and the
band gap. Sensors were fabricated on a glass plate
with silver conducting paste contacts with varying
gaps in the range of 80–250 lm. The photo response
measurements were carried out using Xe arc lamp,
Thermo Oriel monochromator set up and a com-
mercial UV sensor read out by Solartech, Inc. The
experimental set up was calibrated with standard SiC
and AlGaN UV detectors and the output power of
the Xe arc lamp was measured by Newport standard
power meter. The X-ray Photoelectron Spectroscopic
(XPS) measurements were performed using Kratos
Axis 165 spectrometer at a vacuum of 4 · 10
–10
Torr
with non-monochromatic Mg Ka radiation. All bind-
ing-energies were calibrated with respect to C 1s at
284.6 eV.
Results and discussions
Figure 2 shows the photograph of flexible sheets of

ZnO which were used for UV sensor fabrication. SEM
images in Fig. 3(a) and (b) show the morphology of the
ZnO films. The films contain micro spheres in the
range of 600 nm–2 lm embedded in the network of
intricate nanowires. The enlarged images in Fig. 3(c)
and (d) reveal the network of nanowires that are
30–65 nm in diameter and a few microns in length. In
an attempt to understand the process mechanism of
formation of spheres alone in detail, we lowered the
overall growth temperature from 800 °C to 600 °C
(process temperature for the formation of ZnO
microspheres in network of nanowires is 800 °C).
Figure 4(a) and (b) show the SEM of low temperature
Fig. 1 Schematic showing the growth of ZnO nanostructures Fig. 2 The photograph of ZnO nanostructure film
162 Nanoscale Res Lett (2007) 2:161–167
123
processed ZnO in the identical set up. From the
micrograph, it is evident that the size of sphere is
around 2 lm (similar size as in Fig. 3(a)) before the
spheres turn shaping into a perfect hexagon of size
about 5 lm. The formation of radially spherical ZnO
spheres at a lower growth temperature of 650 °C has
been reported and the authors attempted to explain the
evolution of spheres into nanorods when the samples
were annealed to 1000 °C[26]. The radius of the
spheres was calculated to be 400 nm from the formula
proposed by Ding et al. [26];
r
min
¼ 2r

LV
V
L
=
RT ln r
where r
LV
is surface free energy of liquid–vapor, V
L
is
molar volume and r is vapor phase supersaturation.
The inflation of spheres continues with the process as
more vapor gets condensed on to the small spheres. The
mechanism for the formation of nanowires and spher-
ical structure in a one step process (as observed in the
present investigation) in the absence of any catalyst or a
reducing agent is not very clear. However, we believe
that complex nature of DVP process in which the rate
of supersaturation of reactive elements varies, plays the
key role in defining the shape and structure of
the resulting material. Our experiments to understand
the effect of gas kinetics in controlling the shapes are
underway and can be found elsewhere (S. S. Hullavarad
and P.C. Karulkar, under preparation). Mo et al. [27]
have observed formation of ZnO nanorods embedded
in micro hemispheres and spheres by hydrothermal-
thermolysis of Zn(en
2+
) in the presence of long chain
polymer of—poly(sodium-4 styrenesulfonate)—(PSS)

and the authors noted that such formation of multiple
shapes is due to the presence of an appropriate amount
of water soluble long chain polymer. In self catalyst
Fig. 3 SEM of the
nanostructure ZnO thin film
(a) and (b) micro spheres with
a network of nanowires, (c)
and (d) enlarged view of ZnO
nanowires
Nanoscale Res Lett (2007) 2:161–167 163
123
DVP technique, the supersaturation of reactive ele-
ments like Zn vapor and the oxygen favors the for-
mation of nanowires and spheres like morphology.
There have been reports [28] on the fabrication of ZnO
nanowires of diameter 200 nm at a lower growth
temperature of 650 °C and the reaction was carried out
in N
2
atmosphere. The formation of nanowire or
spheres or both of shapes in vapor phase depends on
the temperature, carrier gas and the type of substrates.
It is clear from the two experiments that the nature
of carrier gas used in the vapor deposition process
affects the shape of structures due to difference in
masses of processing gases.
Figure 5 shows the XPS general scan spectrum for
the ZnO nanostructure film consisting of nanowire and
microspheres. The spectrum depicts the core levels at
90.3 eV and 531.5 eV corresponding to Zn 3p and O1s

and Zn Auger lines Zn LMM b, c, d [29]. The ZnO thin
films were investigated by the room temperature PL
spectroscopy. The excitation energy of the laser was
3.6 eV corresponding to a wavelength of 340 nm. As
shown in Fig. 6, the dominant peak was observed at
k = 383 nm which is attributed to the recombination of
free excitons through an exciton–exciton collision
process (D
0
X) corresponding to 3.2 eV [1]. The inset of
Fig. 6 shows the bright blue/violet emission from the
ZnO micro sphere-nanowires network film under laser
illumination. The exciton peak has the full width at half
maximum (FWHM) of 15 nm. Interestingly, the green
emission at k = 512 nm [30, 31] (2.41 eV) is absent in
the PL spectrum indicating the absence of any non-
stoichiometry between Zn and O [32–34].
The nanostructure ZnO film was pressed on to the
silver conducting paste electrode with a defined gap
(Fig. 7). The current–voltage characteristics were car-
ried out with and without illumination as shown in
Fig. 8. The dark (background) current of the nano-
structure ZnO film device was 1 · 10
–10
A at 1 V. This
dark current is comparatively better than the thin film
Fig. 4 (a) and (b) SEM of ZnO micro spheres evolving into
hexagons
200
s1O

dMMLnZ
s1C
cMML
n
Z
bMMLnZ
s3nZ
bMMLnZ
p3nZ
)SPC(ytisnetnI
Binding Energy (eV)
600
400
Fig. 5 XPS general scan of ZnO nanostructure consisting of
microspheres and nanowires corresponding to morphology
shown in Fig. 3
350
1x10
3
2x10
3
3x10
3
4x10
3
Note: No Defect Emission at 512 nm
383 nm
FWHM = 15 nm
)stinu
.bra(

ytisnetnI
Wavelength (nm)
600550
500
450400
Fig. 6 PL spectrum of ZnO nanostructure thin film
164 Nanoscale Res Lett (2007) 2:161–167
123
based UV sensor fabricated from ZnO and MgZnO
UV sensitive materials [35]. A lower dark current is
desirable for a better sensor. When a ZnO nanowire is
exposed to air, the negative space charge layer is cre-
ated when the adsorbed oxygen molecule captures an
electron from the conduction band and therefore the
device exhibits higher resistivity. When the photon
energy is larger than the band gap energy E
g
, the
incident radiation is absorbed in the ZnO nanostruc-
tured UV sensor, creating electron–hole pairs. The
photogenerated, positively charged hole neutralizes
the chemisorbed oxygen responsible for the higher
resistance, increasing the conductivity of the device. As
a consequence, the conductivity in the material
increases, giving rise to photocurrent. The sensor
exhibited a photo current of 1 · 10
–8
A, at 1 V under
UV illumination at 383 nm. The UV to Visible rejec-
tion ratio was found to be two orders of magnitude.

The responsivity R of the UV sensor at 1 V corre-
sponding to a dark current of 1 · 10
–10
A was calcu-
lated to be 50 mA W
–1
from the relation;
R ¼
I
Ph
P
Op
where, the I
Ph
is the photon induced current at
k = 383 nm with the power output P
Op
is the incident
optical power which was measured independently at
the ZnO UV sensor location. Figure 9 shows the photo
response measurements of nanostructure ZnO film
device for varying electrode spacing of 80, 130, 200 and
250 lm. The response for the largest spacing of
250 microns is enhanced 20 times for clarity. The photo
response signal output (mW/cm
2
) was found to increase
with the reducing electrode spacing. The full width at
half maximum (FWHM) of the photo response curves
are measured to be 76, 71, 54 and 55 nm for the elec-

trode spacing of 80, 130, 200 and 250 lm, respectively.
The effect of shapes and sizes of nanostructures on the
photoresponse properties needs to be investigated and
is seldom reported in the literature.
The device response in terms of rise and decay time
at room atmosphere when the UV illumination turned
Glass Plate
ZnO Micro-
spheres
ZnO
nanowires
Ag
conducting
paste
electrode
Fig. 7 The schematic of UV sensor formed from nanostructured
ZnO thin film
-10
10
-12
10
-10
10
-8
10
-6
Photo current
under UV 383 nm
Dark Current
)A( tnerruC

Voltage (V)
1050
-5
Fig. 8 I–V characteristics of ZnO UV sensor with and without
UV illumination
300
130 µm
200
µm
250
µm
80
µm
X 20
)stinu.bra(rewoP
Wavelength (nm)
80 120 160 200 240 280
50
55
60
65
70
75
80
)m
n(M
HWF
Gap Size (µm)
800
700

600
500
400
Fig 9 Photoresponse measurements of UV sensor as a function
of electrode spacing. The inset shows the FWHM of photore-
sponse curves as a function of electrode spacing
0
10
20
30
40
50
60
1.7
3.9
0.5
2.3
41
5.0
55
6.1
250
200
125
80
)sdnoces(emiT
Gap Size (µm)
Rise Time
Decay Time
Fig. 10 The rise and decay time of UV sensor for different

electrode spacing
Nanoscale Res Lett (2007) 2:161–167 165
123
ON and OFF was studied using a modified circuitry that
involves a sensitive oscilloscope. Figure 10 shows the
rise and decay time of ZnO micro sphere-nanowires
network film device under UV illumination. The rise
time was observed to be 6.1, 5, 2.3, 3.9 seconds while
the decay time was observed to be 55, 41, 0.5 and
1.7 seconds for electrodes spacing of 80, 130, 200 and
250 lm, respectively. It is interesting to note that the
rise and decay times are faster for the electrode
spacing of 200 lm whereas the corresponding photo
response was poor. Keem et al. [36] have studied the
photo response of ZnO nanowires grown on pre-
patterned Ti/Au electrodes with a spacing around
50 microns and measured photocurrents with the
trapping time of electrons ranging from 10 ms to
several hours through the nanowires excited by both
the above and below gap light and concluded that the
photo current is surface-related rather than bulk
related. We believe that such faster rise and decay
times observed in our photo response measurements
result from the characteristic structure [37]. A com-
parative study of the effect of oxygen on UV sensors
in the nano and the bulk regime is necessary to
resolve the issues encountered in this study. The
photoconductivity of ZnO and hence its response is
known to depend on the presence of oxygen in the
atmosphere [38]. It is interesting to study the effect of

background oxygen pressure on the rise and decay
time of the UV ZnO nanostructure sensor.
Conclusions
Fabrication of ZnO UV sensors are demonstrated by a
simple route without employing the tedious clean room
procedure of inter-digitized electrode formation. The
active ZnO in the form of a flexible sheet consisted of
micro spheres in a matrix of nanowires. The mor-
phology of the deposited structures indicated that the
sizes of ZnO nano-wires in the range of 30–65 nm and
the micro spheres in the range of 600 nm–2 lm. The
PL measurements indicated the exciton bandgap at
383 nm corresponding to band gap of 3.2 eV. The
photoresponse measurements indicated that the
multiple shapes involving the spheres in network of
nanowires and the electrode spacing affect the sensor
responsivity. This technique has a potential for scale up
to fabricate UV sensors for mass production.
Acknowledgements SSH is thankful to Dr. Diane Pugel and
Dr. R.D. Vispute for fruitful discussions. Authors would like to
acknowledge the support from Defense Micro Electronic
Agency (DMEA) at University of Alaska, Fairbanks.
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